Microphone-Reciprocity Calibration

Abstract

A simplified technique of performing a closed-coupler (cylindrical cavity) reciprocity calibration of microphones is discussed All three transducers needed in the calibration procedure remain as parts of the coupler throughout the measurement. A multiple-function switch is used to connect the circuits for the required operations in sequence, without physically interchanging transducers. The same switch operates an analog computer, which uses a logarithmic potentiometer to solve the necessary calculations. The auxiliary transducer is a piezoelectric ceramic ring, which makes up the cylindrical wall of the cavity; thus, difficulties associated with having all three transducers in the coupler at the same time are avoided. Considerations involved in using this piezoelectric ring are discussed. Additional features of the calibration technique are (1) use of a capacitor in the measurement of the driving current of the reciprocal transducer, and (2) use of a resistive-attenuator-insert technique in the measurement of open-circuit voltage ratios. The calibrator is designed for calibration of a front-damped piezoelectric-ceramic microphone, and the problems resulting from the front-damping are discussed.